It has been the practice to employ redundant equipment units, e.g., plug-ins, circuit packs and the like, to enhance product reliability. The enhanced reliability has been realized by controllably switching service to a standby equipment unit upon detection of a failure of an in-service equipment unit or its removal from a connector in an equipment frame.
Prior known equipment unit protection switching arrangements that were specifically directed toward removal of equipment units, relied upon the complete disengagement of the equipment unit from its connector before initializing an equipment unit protection switch. Any movement of an equipment unit in its connector tends to cause transmission errors which have the potential for affecting customer service. This is especially true in relatively high digital bit rate transmission arrangements.
In such prior equipment unit protection switching arrangements, the time required to effect an equipment unit protection switch is directly dependent upon the speed at which the equipment unit is being removed from its connector. If the equipment unit was removed quickly, the protection switch would occur sooner than if it was removed slowly. In any event, the interval is still too long before completion of the equipment unit protection switch. Prior attempts at minimizing the time required to complete an equipment unit protection switch were primarily directed toward the use of complex fault detection and isolation algorithms. Such prior algorithms required a significant amount of processing time and processing resources.